Re: [PATCH] Unbuffer stdout and stderr on windows

[Yao Qi <yao@codesourcery.com>]

On 08/28/2013 04:39 AM, Pedro Alves wrote:
> Better say that "it is not used on Windows hosts".  But I think
> it'd be better even to say:
> 
>    /* Method 'to_write_async_safe' is not overwritten, because
>       there's no way to flushing a stream in an async-safe manner.
>       Fortunately, it doesn't really matter, because:
>         - that method is only used for printing internal debug output
> 	 from signal handlers.
>         - Windows hosts don't have a concept of async-safeness.  Signal
> 	 handlers run in a separate thread, so they can call
>           the regular non-async-safe output routines freely.  */

OK.

[...]
> hide that in ui-file.c itself.  So e.g., we'd have a new stderr_fileopen
> method, and then both places that create gdb_stderr would use it, like:
> 
> -gdb_stderr = stdio_fileopen (stderr);
> +gdb_stderr = stderr_fileopen ();
> 
> We could also that whole set of Windows-specific comments there
> too.

Right.  Function stderr_fileopen is added in the updated patch.
What do you think?

-- 
Yao (齐尧)

gdb:

2013-08-28  Yao Qi  <yao@codesourcery.com>
	    Pedro Alves  <palves@redhat.com>

	* event-top.c (gdb_setup_readline): Call stderr_fileopen
	instead of stdio_fileopen.
	* main.c (captured_main) [__MINGW32__]: Set stderr unbuffered.
	Call stderr_fileopen instead of stdio_fileopen.
	* ui-file.c (stderr_file_write): New function.
	(stderr_file_fputs): New function.
	(stderr_fileopen): New function.
	* ui-file.h (stderr_fileopen): Declare.
---
 gdb/event-top.c |    2 +-
 gdb/main.c      |    9 ++++++++-
 gdb/ui-file.c   |   54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 gdb/ui-file.h   |    3 +++
 4 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/gdb/event-top.c b/gdb/event-top.c
index f00ab7d..f1d55b3 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -955,7 +955,7 @@ gdb_setup_readline (void)
      time.  */
   if (!batch_silent)
     gdb_stdout = stdio_fileopen (stdout);
-  gdb_stderr = stdio_fileopen (stderr);
+  gdb_stderr = stderr_fileopen ();
   gdb_stdlog = gdb_stderr;  /* for moment */
   gdb_stdtarg = gdb_stderr; /* for moment */
   gdb_stdtargerr = gdb_stderr; /* for moment */
diff --git a/gdb/main.c b/gdb/main.c
index 1c240e4..11f4b03 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -375,8 +375,15 @@ captured_main (void *data)
   saved_command_line[0] = '\0';
   instream = stdin;
 
+#ifdef __MINGW32__
+  /* Ensure stderr is unbuffered.  A Cygwin pty or pipe is implemented
+     as a Windows pipe, and Windows buffers on pipes.  */
+  setvbuf (stderr, NULL, _IONBF, BUFSIZ);
+#endif
+
   gdb_stdout = stdio_fileopen (stdout);
-  gdb_stderr = stdio_fileopen (stderr);
+  gdb_stderr = stderr_fileopen ();
+
   gdb_stdlog = gdb_stderr;	/* for moment */
   gdb_stdtarg = gdb_stderr;	/* for moment */
   gdb_stdin = stdio_fileopen (stdin);
diff --git a/gdb/ui-file.c b/gdb/ui-file.c
index cf5a86d..d29b8da 100644
--- a/gdb/ui-file.c
+++ b/gdb/ui-file.c
@@ -654,6 +654,60 @@ stdio_file_fseek (struct ui_file *file, long offset, int whence)
   return fseek (stdio->file, offset, whence);
 }
 
+/* This is the implementation of ui_file method to_write for stderr.
+   gdb_stdout is flushed before writing to gdb_stderr.  */
+
+static void
+stderr_file_write (struct ui_file *file, const char *buf, long length_buf)
+{
+  gdb_flush (gdb_stdout);
+  stdio_file_write (file, buf, length_buf);
+}
+
+/* This is the implementation of ui_file method to_fputs for stderr.
+   gdb_stdout is flushed before writing to gdb_stderr.  */
+
+static void
+stderr_file_fputs (const char *linebuffer, struct ui_file *file)
+{
+  gdb_flush (gdb_stdout);
+  stdio_file_fputs (linebuffer, file);
+}
+
+/* Create a ui_file from stderr.  */
+
+struct ui_file *
+stderr_fileopen (void)
+{
+  struct ui_file *ui_file = stdio_fileopen (stderr);
+
+#ifdef __MINGW32__
+  /* There is no real line-buffering on Windows, see
+     http://msdn.microsoft.com/en-us/library/86cebhfs%28v=vs.71%29.aspx
+     so the stdout is either fully-buffered or non-buffered.  We can't
+     make stdout non-buffered, because of two concerns,
+     1.  non-buffering hurts performance,
+     2.  non-buffering may change GDB's behavior when it is interacting
+     with front-end, such as Emacs.
+
+     We decided to leave stdout as fully buffered, but flush it first
+     when something is written to stderr.  */
+
+  /* Method 'to_write_async_safe' is not overwritten, because there's
+     no way to flushing a stream in an async-safe manner.
+     Fortunately, it doesn't really matter, because:
+     - that method is only used for printing internal debug output
+       from signal handlers.
+     - Windows hosts don't have a concept of async-safeness.  Signal
+       handlers run in a separate thread, so they can call
+       the regular non-async-safe output routines freely.  */
+  set_ui_file_write (ui_file, stderr_file_write);
+  set_ui_file_fputs (ui_file, stderr_file_fputs);
+#endif
+
+  return ui_file;
+}
+
 /* Like fdopen().  Create a ui_file from a previously opened FILE.  */
 
 struct ui_file *
diff --git a/gdb/ui-file.h b/gdb/ui-file.h
index 9fef68c..dca800f 100644
--- a/gdb/ui-file.h
+++ b/gdb/ui-file.h
@@ -129,6 +129,9 @@ extern struct ui_file *mem_fileopen (void);
 /* Open/create a STDIO based UI_FILE using the already open FILE.  */
 extern struct ui_file *stdio_fileopen (FILE *file);
 
+extern struct ui_file *stderr_fileopen (void);
+
+
 /* Open NAME returning an STDIO based UI_FILE.  */
 extern struct ui_file *gdb_fopen (const char *name, const char *mode);
 
-- 
1.7.7.6